Great lead. Using mnt_id as an index into /proc/[pid]/mountinfo would give the mountpoint directory. How to efficiently perform this check for all processes (irrespective of name)?
– Tom HaleAug 18 '17 at 8:50

1

man proc(5) says sdev: The ID of the device where the target file resides (in hexadecimal). It also explains mnt_id and the mountinfo file.
– Tom HaleAug 18 '17 at 8:59